Top Quark Measurements at the ILC Akimasa Ishikawa

Top Quark Measurements at the ILC Akimasa Ishikawa (Tohoku University)

Introduction • Top quark is the heaviest elementary particle. – mt = 173. 1 ± 0. 9 Ge. V • The mass is close to EW scale v/√ 2 = 174 Ge. V (yt ~ 1) – Top quark may play an important role in EW symmetry breaking • Top quark decays before hadronization – Lifetime is about 5 x 10 -25 s – Can probe bare quarks

Top Quark at Hadron Colliders • Top quarks are only studied at hadron colliders. – Top quarks are produced from QCD reaction, dominantly quark pairs annihilation at the Te. Vatron or gluon fusion at the LHC. – Initial state energy/polarization is unknown – Theoretical uncertainty in QCD is large

Measurements at Hadron Colliders • The properties are partially known – Mass in pole “like” scheme which is hard to translate to mass in Msbar scheme : mt = 173. 1 ± 0. 9 Ge. V • From cross section : mt. MSbar = 160 +- 5 Ge. V – – Width with indirect method : Gt = 2. 0 +- 0. 5 Ge. V Spin and charges Coupling to gluon (to axigluon at the Te. Vatron? ? ) |Vtb|

Top quark at the ILC • Top quarks are produced from electroweak reaction – Theoretically clean – Experimentally easier to reconstruct • Tunable Beam energy and polarized beam – Threshold energy scan is possible – Chiral structure can be tested

Threshold Scan • The ttbar cross section is functions of several parameters • By scanning the threshold region, those parameter can be determined 9% enhancement Uncertainty of s ~4% ar. Xiv: hep-ph/9804241 Y. Kiyo We can measure top yukawa before going to Ecm=500 Ge. V 6

Experimental Cross Section Luminosity spectrum is not d function ! Th Cross section Need to convolute theoretical cross section with luminosity spectrum After Convolution Luminosity spectrum @350 Ge. V Experiment(toy MC) Convoluted theory 22 a. SD-5 2021/3/6 7

Assumptions for Threshold Scan •

Signal and Backgrounds Signal 6 -Jet Branching Fractions 4 -Jet 6 -Jet 45% 4 -Jet 44% 2 -Jet 11% Backgrounds e+e- WW, ZZ, ZH “LR” pol. “RL” pol. ttbar WW ZZ ZH 22 a. SD-5 2021/3/6 9

Top Yukawa • Enhancement due to Higgs exchange is 9% • Almost no Ecm dependence Y. Kiyo

Sensitivity to Top Yukawa : Case A Stat error 6 -Jet (Left) 6 -Jet (Right) 4 -Jet (Left) 4 -Jet (Right) Cross section 0. 8% 1. 2% 0. 9% 1. 3% Top yukawa 5. 0% 7. 2% 5. 1% 7. 9% Combined ALL 3. 0%

Fits to Mass and Width : Case A Clear discrimination of 200 Me. V differences for mass and width

Results on Mass and Width Input s mt. PS = 172 Ge. V Gt = 1. 4 Ge. V 6 -Jet 4 -Jet combined Stat error (Me. V) Left (110 fb-1) 23 29 24 30 --- Right (110 fb-1) 34 42 33 42 --- L+R (220 fb-1) 20 24 19 25 14 17 Stat error of top mass and width are 14 Me. V and 17 Me. V 22 a. SD-5 2021/3/6 13

Fits to Top mass and as : Case B PDG as = 0. 1184 ± 0. 0007 (0. 6%)

Vacuum Stability Current Status • Vacuum stability can be discussed with top mass and Higgs mass • Our vacuum might be meta-stable from current world averages of top mass and Higgs mass in the SM! – top mass from cross section • But the uncertainties on masses are large so we can not conclude the fate of our universe.

Vacuum Stability Future • Systematic error should be considered – Luminosity spectrum < 100 Me. V? ? – Theoretical uncertainty ~ 100 Me. V DMH = ± 37 Me. V Dmtpole = ± 17 Me. V Only Stat error. 22 a. SD-5 2021/3/6 16 Stat error of Higgs mass is by Watanuki with recoil with μμh

Form Factor Measurements • In Warped Extra Dimension model (bulk RS 1), wave functions of heavy particles are close to IR brane while wave functions of light particles are localized at UV brane. • Heavy top mass is explained by an overlap of right handed top quark and Higgs wave functions in the 4 th spatial dimension direction. – Couplings of left and right handed tops to Z is different • In some composite models, Higgs and top quark are composite. • These can be searched with form factor measurements

Form Factors • Only the SM are non-zero in • are dipole moment form factors. • Modified vertex could explain the discrepancy of AFB 0, b at LEP • Interference of g and Z allows a determination of relative sign. – Polarization are very powerfull tool – It is difficult at the LHC where form factors are measured from tt. Z and ttg

Angular Analysis • s(Ecm, qtop, qhel) • Top quark charge is measured with lepton (4 jet+lepton final states) • Polar angle of top quark can be measured with very small bias. – Forward-Backward Asymmetry • Helicity angle of top quark is also measured with small bias at +-1 that can be easily corrected. – Determination of a fraction of t. L and t. R – Right handed electron t. L enriched

CP Conserving Couplings • We assume Ecm = 500 Ge. V and 500 fb-1 – About 100, 000 top pair events. • From, differential cross section, CP conserving couplings are extracted • All couplings are measured less than 1% precisions – Which new physics parameter space is excluded? is fixed to zero

CP Violating Couplings • If 125 Ge. V is CP mixture states, a few % CP Violating couplings are possible at Ecm~370 Ge. V. • Has not been done at the ILC – Roman Poeschel, Yuichiro Kiyo et al started the analysis.

Summary • From a threshold scan – Measure top quark mass with 14 Me. V stat error which draws a definitive conclusion of vacuum stability in the SM • Systematic uncertainty is ~100 Me. V? • And higgs mass less than 50 Me. V – Measure top quark width with 17 Me. V stat error • Allows to search for anomalous couplings – Measure top yukawa before going to Ecm=500 Ge. V, tt. H production. • At higher energy, – Stat error of CP conserving form factors are estimated which is much better than LHC – One question to theorists. Which new physics parameter space is excluded?

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CP Violating Couplings at TESLA • But done at the TESLA